Alkaline dry cells are conventionally used in low-voltage power sources of strobes, shavers, table clocks, portable acoustic equipment and electronic equipment and so forth. Positive electrode cans used in such cells are those manufactured by punching out a cold-rolled steel sheet or a Ni-electroplated steel sheet formed using the cold-rolled steel sheet as a material and working the resulting sections by multiple drawing into desired forms. However, positive electrode cans made of such materials have so poor a corrosion resistance that, when their inside surfaces come into contact with high-concentration electrolytes and positive electrode mixtures, oxide layers are formed on the surfaces of the positive electrode cans to cause a lowering of cell characteristics such as short-circuit current and electromotive force as time lapses.
Accordingly, as a countermeasure for such a problem, a method has been proposed in which the inside surface of a positive electrode can is coated with a conductive coating composition mixed with a carbon type conductive agent in a large quantity of, e.g., 100 to 300 parts by weight based on 100 parts by weight of resin so that its corrosion resistance can be improved. This is a method in which a carbon type conductive agent as exemplified by graphite powder or acetylene black is mixed into a coating composition comprising a resin with a good alkali resistance such as polyvinyl isobutyl ether or vinyl acetate resin and the resulting coating composition is applied to the inside surface of a positive electrode can (Japanese Patent Applications Laid-open No. 58-48361 and No. 59-160959).
Application of this conductive coating composition on positive electrode cans, however, must be carried out by a very cumbersome operation of spray-coating or brush-coating positive electrode cans one by one, followed by drying, and hence has had a poor productivity. In addition, since the positive electrode cans are small in diameter and yet deep, it has been very difficult from a technical viewpoint to uniformly coat the inside surfaces. Accordingly, there has been a demand for coated steel sheets from which positive electrode cans with a good quality can be efficiently manufactured by only machining.
To meet such a demand, the present inventors have proposed a coated metal sheet for dry cell positive electrode cans, comprising a metal sheet having a chromate film optionally formed thereon, and covered thereon with a coating film of a coating composition in which one or both of graphite powder and carbon black is/are mixed so that conductivity and workability can be balanced (Japanese Patent Application Laid-open No. 6-342653, U.S. Pat. No. 5,527,641, European Patent Application No. 94 909 315.7-2111).
This coated metal sheet comprises a metal sheet having a chromate film and covered thereon with a coating film mainly composed of at least one of an epoxy resin, an epoxy-acrylic resin and a polyester resin, containing a conductive agent comprising at least one of graphite powder having a purity of not less than 95%, having been made flaky to have an aspect ratio of not less than 50 and having an average particle diameter of not larger than 50 .mu.m and carbon black having an oil absorption of not less than 250 ml/100 g as measured using DBP, or a coating film further incorporated with a cross-linking agent; the graphite powder and carbon black being contained in an amount falling in the range defined by straight lines connecting the respective proportions (including straight lines) of 20:0 and 100:0, 100:0 and 50:15, 50:15 and 0:20, 0:20 and 0:5, and 0:5 and 20:0, as graphite powder:carbon black proportions in parts by weight indicated by rectangular coordinates, and positive electrode cans can be manufactured only by machining.
However, when as this coated metal sheet a coated steel sheet comprised of a Ni-electroplated steel sheet obtained by applying single-layer Ni-coating on an ordinary cold-rolled steel sheet is used and the coated steel sheet is worked into positive electrode cans by pressing, a work stress due to differences in workability of the coating film and the steel sheet may be produced at the interface between the both to cause a lowering of coating adhesion. If alkaline dry cells are fabricated using positive electrode cans having such a disadvantage, the strongly alkaline electrolyte may enter in the interface between the coating film and the steel sheet to oxidize the steel sheet surface, resulting in a deterioration of cell characteristics with time.
Ni-electroplated steel sheets formed of cold-rolled steel sheets are conventionally used to manufacture cans of various types. When cans having a surface coating are manufactured, coated steel sheets are not used, but uncoated steel sheets are worked into cans, followed by printing to apply coating on the surface. Thus, almost no technical data relating to the workability of coated steel sheets comprised of Ni-electroplated steel sheets are available in the past, and no attempt has been made on improvements in the coating adhesion which may lower because of work stress.
An object of the present invention is to provide a coated steel sheet comprised of a Ni-electroplated steel sheet that may cause no lowering of coating adhesion due to work stress even when worked into alkaline dry cell positive electrode cans by pressing.